In motor vehicles with an internal combustion engine, auxiliary units, such as an air conditioning compressor or a generator, are driven via the crankshaft. By means of a vibration damping device of the type mentioned in the introduction, the rotary movement of the crankshaft is then transferred onto the auxiliary unit via a belt drive.
Due to the rotational irregularities of the crankshaft, vibrations are introduced into the belt drive. In order to isolate these vibrations, a decoupling device is disposed between the hub and the belt pulley. Such vibration dampers are also referred to as decoupled belt pulleys or decoupled torsional vibration dampers. The torsional rigidity of such a decoupling device is most frequently selected in such a way that the first excited natural torsional frequency of the system auxiliary unit-drive pulley is sufficiently below the idling speed of the internal combustion engine.
One example for a vibration damping device with a decoupling device is apparent from DE 197 49 761 C2. The decoupling device is configured as a soft elastomer spring that connects the hub and the belt pulley with each other. In this case, the elastomer spring decouples, or isolates, the vibrations acting from the crankshaft on the belt pulley.
The lower the damping effect of the decoupling device can be set, the more effective the decoupling effect of the decoupling device is in the speed range of the internal combustion engine. However, this is disadvantageous particularly in internal combustion engines with a start-stop system because it is necessary to pass through the first resonance of the system auxiliary unit-belt pulley during each start-stop process of the internal combustion engine. Thus, the decoupling device which, due to its function, is soft and low-damping, is subjected to very large resonance amplitudes. The coupled masses of the belt drive are often incapable of following them, so that there is an occurrence of belt slip in conjunction with unacceptable noise and belt wear.
To avoid these drawbacks, DE 10 2004 035 969 A1 describes a vibration damping device with a decoupling device comprising an elastomer spring and a freewheel member connected in series therewith.